1. Field of the Invention
The present invention relates generally to DC voltage reference circuits, and more specifically, to low-noise voltage reference circuits and techniques that do not require large filter capacitors.
2. Background of the Invention
Voltage references are needed in most mixed-signal applications such as digital-to-analog converter (DAC) circuits and analog-to-digital converter (ADC) circuits, among others. In order to provide a low-noise and precise voltage reference, an indicator of the value of the DC output voltage of the reference is typically compared to another reference voltage, such as the output voltage of a bandgap circuit. In order to make such a comparison, and in order to provide a low noise reference voltage output in general, the DC output voltage of the reference is typically filtered to remove noise and other variations such as power supply ripple that affect the instantaneous value of the DC output voltage. The filter is typically a capacitor (or multiple parallel capacitors) having a low equivalent series resistance (esr), and often, an external terminal is required for integrated circuit (IC) voltage references, so that a suitable filter capacitor can be provided external to the IC die.
Terminals are always at a premium in IC designs and may determine the size and cost of the IC due to package selection. Also, an external capacitor adds cost to the circuit. Alternatively, if a suitable capacitor can be provided on a die, the real estate required usually will burden the design and is therefore undesirable.
Therefore, it would be desirable to provide a low-noise voltage reference that does not require a large internal or external filter capacitor for low noise operation.